The fluidized bed incinerator is known for incinerating and disposing of refuse such as municipal waste. The method of incineration and disposition of the refuse in the fluidized bed incinerator is to burn the refuse while fluidizing it with air inside the incinerator. In order to improve the fluidization and combustion of the refuse, a fluidizing medium such as sand is fed together with the refuse into the fluidized bed.
A typical type of fluidized bed incinerator is equipped with a number of air diffuser tubes or air diffuser plates (called "air diffusers" hereinafter) for blowing air into the lower section of the incinerator body, and further, the upper section of the incinerator body is equipped with a refuse feeding unit and a fluidizing medium feeding unit.
The refuse is burned while both the refuse and the fluidizing medium are deposited onto the air diffusers inside the incinerator body where they are fluidized by the primary air blown from the air diffusers.
The refuse generally contains a variety of materials such as low calorie refuse such as discarded food, high calorie refuse such as plastics and rubber, refuse such as shredded or chipped furniture, or refuse such as fragmented metallic or vitreous containers, bottles or cans.
As the refuse is fed to the fluidized bed, the combustibles are burned, of which plastics and similar substances are melted by heat to generate pyrolysis gas, and the incombustibles such as metal and glass remain unburned (called "combustion residue" hereinafter).
As the fluidizing medium is gradually fed to the fluidized bed, a moving bed of fluidizing medium descends. Therefore, while the combustibles are being burned or decomposed within the fluidized bed, the combustion residue, together with the fludizing medium, descends down through the gaps between the air diffuser tubes at the lower section of the fluidized bed. The fluidizing medium is then separated from combustion residue, and is again fed to the fluidized bed.
Secondary air is supplied to the upper section of the fluidized bed, where the generated pyrolysis gases are burned. Because, in this type of fluidized bed incinerator, the sand which is deposited onto the fluidized bed as the fluidizing medium is oscillated while it descends and is heated, the agitation and dispersion of the refuse is promoted.
For this reason, the refuse deposited onto the fluidized bed is dispersed uniformly in the presence of fluidizing medium and is dried, ignited, decomposed, and burned instantaneously. Further, the ash and dust produced therein are drawn along with the fluidizing air, out of the upper section of the incinerator and are collected by an electric precipitator.
Consequently, the refuse deposited onto the fluidized bed is disposed of almost completely, leaving behind some metallic, vitreous, or ceramic residue. The ratio of these substances to the refuse is usually 2%, and therefore a fluidized bed incinerator can dispose of 98% of the refuse.
A primary advantage of the fluidized bed is that it can reduce the volume of combustion residue to 1/3 compared to a conventional mechanical incinerator such as a stoker-type incinerator.
A problem exists, however, with fluidized bed incinerators in that the refuse deposited onto the fluidized bed is burned and decomposed at a speed so high that the refuse cannot be stably combusted. The refuse has different caloric value depending on the particular content of refuse, and it is often difficult to supply a constant volume of the refuse to the fluidized bed. For example, suppose that a large amount of the refuse is deposited at the same time onto the fluidized bed; in this event, a large quantity of pyrolysis gas and smut are also generated simultaneously even though the refuse is burned and decomposed instantaneously. In this instance, not only is it impossible to completely secondarily combust the resulting large quantity of pyrolysis gas with secondary air inside the incinerator, but it is also difficut to entirely collect the large quantity of smut contained in the exhaust gas by means of an electric precipitator.
The principal object of this invention is to provide a method for burning and decomposing the refuse slowly inside the fluidized bed incinerator, for secondarily combusting the generated combustible gases in the upper section of the incinerator, for improving the combustion of the mixture of combustible gas and secondary air, and for maintaining the temperature of the combustible gas in the incinerator at a high level.
An additional object is to slow the combustion of refuse in the fluidized bed by injecting the secondary air into the combustion chamber in order to carry out secondary combustion.